1. Field of the Invention
This invention relates generally to improved geophones and other motion sensors. More specifically, this invention relates to a geophone or similar motion sensor that has improved coupling and is less sensitive to electrical noise.
2. Related Art
Typically, geophones comprise a heavy, rugged external case with a spike on the bottom that allows the geophone to be coupled to a reference surface, such as the ground. Within the external case is a mass that is relatively light with respect to the external case, sometimes referred to as a “proof mass.” The proof mass is commonly coupled to the external case by means of a relatively soft spring. Thus, the external case moves with the ground or reference surface, but the proof mass generally does not move with the external case. This creates a relative motion between the proof mass and the external case. This relative motion is detected because the external case is generally magnetized and there is a coil within the proof mass in which current is induced by the movement of the proof mass through the magnetic field. In a typical application, the voltage within the coil is proportional to the relative velocity between the external case and the proof mass. This voltage can then be processed in order to determine the relative velocity.
Because the geophone senses the relative motion between the external case and the proof mass, an important factor in ensuring accurate measurements by a geophone or other ground motion sensor is to ensure that there is a sufficiently high quality of coupling between the geophone (and in particular, the external case) and the reference surface. It is desirable for the movement of the external case to track the movement of the reference surface as closely as possible.
Poor coupling can occur for various reasons. For example, when a geophone is disposed on the ground, the mass of the geophone and the compliant properties of the soil can affect the accuracy of motion measurements. This is because the top layer of the soil is influenced by the mass of the geophone and the motion of that layer with the geophone on top of it will differ from the movement of that layer in the absence of the geophone. Thus, the geophone will not accurately measure the motion of the reference surface, but only the motion of the reference surface as influenced by the geophone itself.
Because of these problems with coupling, many existing geophones cannot be deployed by simply throwing or dropping the device onto the ground or reference surface. In addition, most geophones are too delicate to maintain functionality when they are deployed in this way.
Thus, a need exists for a geophone or similar sensor that has improved coupling characteristics such that the geophone can be successfully deployed by simply throwing or dropping the geophone onto the reference surface. Moreover, it would be particularly desirable to have a geophone that is sufficiently rugged to maintain functionality after being deployed in this fashion. Finally, it would be desirable for such a geophone to have reduced electrical background noise such that it would be capable of detecting relatively weak ground motion signals.